[help] The Shop Item Is Not Recognized

So I started this ranch selling horse eggs. For the 3th time I have the complain that it doesn't work propper. People get after buying one "The Shop Item Is Not Recognized" all the time.

So guess I'm doing something wrong? I have on sign [slot 1-27]. I place eggs on stats, not race. Does it has to be the same race if you put [slot 1-27]?I saw somebody else who had a chest that gave random breed. How can I fix that as well? Many thanks in advance!

What I did when I once sold horse eggs, is I filled 26 of the slots with dirt, and put a horse egg into the 27th slot. I set up hoppers into the back of the chest to auto-refill the horses when they were bought. On the fourth line of the sign, put "[Slot 27]", and that should fix your problem.